The present invention relates to a rangefinder in which trigonometric rangefinding is utilized.
Such a rangefinder comprises a light source S for projecting a light beam on a target object existing in any one of distance zones .alpha., .beta., .gamma. or .delta. as shown in FIG. 1. A plurality of light receiving elements R.sub.A, R.sub.B, R.sub.C and R.sub.D, (such as photo sensitive diodes), are provided, each of which is assigned to the normal distance zones A, B, C and D and are placed at positions apart from said light source S. The respective distances therebetween are defined by the principle of trigonometric rangefinding to enable the light receiving elements to receive an image light spot reflected from the target object Q and to thereby enable said light receiving elements to produce signals indicative of the distance between the target object and the rangefinder.
Such a rangefinder is disclosed in U.S. Pat. No. 3,723,003.
In the conventional rangefinders of this type, the light receiving elements and processing circuit for processing the signal fed therefrom are arranged in such manner that an image light spot reflected from the target object is projected on only one of the light receiving elements at a time to produce signals indicative of one of the distance zones .alpha. through .delta..
However, in practice, the image light spot is apt to be formed over two or three light receiving elements by the effect of the spreading of the projected light beam caused by lens aberration and the spreading of image light spot caused by the image forming lens disposed in front of the light receiving elements. These defects are particularly noticeable when the target object exists on or near the border line between the two adjacent normal distance zones.
When the defects described above occur, the conventional rangefinder can not detect the distance zones accurately. In order to obviate such troubles, it has been suggested that either the length of respective distance zones assigned to corresponding light receiving elements be enlarged or the base length between the light source and the light receiving elements be extended. In the former case, however, the rangefinding precision becomes poor and in the latter case, the rangefinding device becomes large in size and occupies a large space.
To the contrary, it is required that a rangefinder should be made not only as small as possible but also be made to precisely detect the distance zones when it is installed in a camera.
In order not only to minimize the size of the rangefinder but to precisely detect distance zones, the use of an extremely fine light beam and highly precise optical system has been proposed; however, such a light source and optical system are expensive.